Preheat Oven: Preheat oven to 350°F (175°C). Line a muffin tin with paper liners or grease lightly.
Mash Bananas: In a bowl, mash the ripe bananas until smooth.
Mix Wet Ingredients: Add melted butter, egg, and vanilla to the bananas. Whisk until combined.
Combine Dry Ingredients: In a separate bowl, mix flour, sugars, baking soda, baking powder, salt, and cinnamon.
Combine Mixtures: Gently fold the dry ingredients into the wet mixture until just combined. Avoid overmixing.
Add Mix-ins (Optional): Fold in walnuts or chocolate chips if using.
Fill Muffin Tin: Divide batter evenly into the muffin cups (about ¾ full).
Bake: Bake for 18-22 minutes or until a toothpick inserted comes out clean.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack.